Up-converting phosphor is a type of material that emits visible light when excited by long wavelength light. Ytterbium/erbium co-doped fluoride is a high efficiency up-converting phosphor that emits strong green fluorescence and relatively weak red fluorescence when excited by 946-970 nm infrared light.
Due to their unique properties, up-converting phosphors are used in the fabrication of light-emitting diodes, solid-state lasers, and as ultra-sensitive fluorescent labels in biological detections (U.S. Pat. No. 5,674,698).
As a fluorescent label for biological molecules, the phosphor must be small in size and uniform, highly luminescent, and stable in aqueous environment. We have disclosed a method of preparing nanometer-sized molybdenum up-converting phosphor particles (Patent Number: CN 01134861.5). With this method, particles of 50-60 nm in diameter were prepared. They were easy to make, and highly luminescent. But the range of particle size was limited to 50-60 nm. And the annealing temperature is high. James Kane has disclosed in U.S. Pat. No. 5,891,361 and WO 97/46488 a process for preparing small up-converting fluoride phosphor particles. The process involves high temperature (750° C.) heating, and the size of the particle (about 400 nm) thus obtained is too large for many biological applications.